[client] spice: own transport configuration

Register SPICE connection and component options in the SPICE transport.
Move generic input and clipboard settings out of the SPICE namespace,
while retaining migration aliases for existing configurations.

Keep PureSpice and usbredir build requirements private to the transport,
and remove obsolete SPICE configuration state from the client core.

client/src/main.h

@@ -204,13 +204,6 @@ struct AppParams
   bool                 setGuestRes;
   int                  fpsMin;
   LG_RendererRotate    winRotate;
-  bool                 useSpice;
-  bool                 useSpiceInput;
-  bool                 useSpiceClipboard;
-  bool                 useSpiceAudio;
-  bool                 useSpiceUSBAudio;
-  const char *         spiceHost;
-  unsigned int         spicePort;
   bool                 clipboardToVM;
   bool                 clipboardToLocal;
   bool                 scaleMouseInput;